Many researchers, including the inventor herein, have spent many years trying to understand the biophysical mechanisms of the human ear to understand how the human ear works and also to help in developing a module, which may be physically constructed, which duplicates or simulates the operation of the ear. Many theories and models have been developed over the years which have progressed this development. The inventor herein has previously developed a biophysical model (Goldstein, J. L. "Modeling rapid waveform compression on the basilar membrane as multiple-bandpass-nonlinearity filtering", Hearing Research 49, 39-60) which sought to explain the biophysical mechanisms of the ear in terms of a multiple band pass non-linear input/output model for simulating the non-linear basilar membrane response. The approach in this prior model was to account for non-linear cochlear phenomena directly in terms of a mathematical model of I/O behavior rather than treating them as non-linear perturbations from a physically based linear theory. This basic model, and later models, incorporate two non-linearly interacting filter systems that simulate the two distinct non-linear regimes observed in all non-linear phenomena at low-to-moderate and moderate-to-high stimulus sound levels. While this prior model and theory represented a dramatic departure from the prior art in itself, and helped to advance the work on understanding the ear as it provided new direction, this model was not a complete answer in that it did not explain observed phenomena such as distortion products and otoacoustic emissions, or otherwise provide any understanding of the operation of the organ of Corti. Furthermore, the model did not fully address the function of the basilar membrane as the model structure does not reflect the known biophysical properties of the cochlea.
With the present invention (See Goldstein, J. L., Changing Roles in the Cochlea: Bandpass Filtering by the organ of Corti and Additive Amplification of the Basilar Membrane, paper 4aPP3 at 124th Meeting of the Acoustical Society of America, November 1992), the inventor has improved on his earlier model to bring it further in congruence with the known biophysical mechanisms of the ear, including most particularly the organ of Corti and basilar membrane, and has in the process achieved simulation of the heretofore unexplained distortion product and otoacoustic emissions. In an intermediate step, the inventor has added bilateral signal processing to his prior model to more completely simulate the two signaling channels responsible for the "tips" and "tails" of Cochlear tuning curves. This bilateral processing potentiates extension of the model to other phenomena, including combination tones (distortion products) and otoacoustic emissions. This intermediate step takes advantage of non-linear feedback while the full invention adds distributed amplification. This distributed amplification provides for the non-linear addition of many signals from tip sources which are believed to function similarly to the organ of Corti. These organ of Corti filters, or tip sources, are connected at different locations along a filter-bank spectrum analyzer (a corollary to the outer hair cells and adjoining structures) and are non-linearly added through a propagating medium (a corollary to the basilar membrane) to provide distributed amplification. This model thus helps explain the non-linear input/output characteristic as observed by others in the basilar membrane mechanical response in the human ear.
There are several additional features of the present invention, including a "zoom" capability, a sensitivity adjustment, and efferent neural control simulation. As explained in greater detail below, the present invention includes a pair of matched all pole lattices with a plurality of tip couplers tapped into each lattice and interconnecting them at chosen "center frequencies". A scaling factor, or alpha, may be induced at any frequency to alter the response at that frequency and thereby match the model's output to any particular human ear output. Additionally, an efferent bias control, which is ordinarily set to zero, may also be used to scale the throughput of any one or more tip couplers to simulate the brain's ability in humans to "tune out" undesirable sounds or simulate "listening without hearing" as experienced in humans. Choosing the number of tip couplers (and hence the length of the matched lattices), and the "center frequencies" of each of the tip couplers permits the model builder to focus on any one or more range of frequencies for measurement with the model. Additionally, the model accommodates the use of 12,000 tip modules which corresponds to the full complement of outer hair cells believed to be contained and operative in the organ of Corti, to thereby provide a full representation and simulation of the frequency range of the human ear. As this may be cumbersome or undesirable, a fewer number of tip couplers may be used and may be focused over a chosen portion of the frequency range of hearing to thereby minimize cost and complexity of the model while still simulating with great accuracy the desired response frequencies.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.